Typically, radar apparatuses are equipped with an antenna which emits (transmits) an electromagnetic wave at a predetermined frequency in response to supply of emission electric power and receives the electromagnetic wave from the outside, such as a reflection wave of the transmitted wave. An antenna device is constituted by the antenna and a radome having a shape to cover the antenna so that it protects the antenna from the external environment.
The radome has a wall surface also in the emitting direction of the antenna because it is a structure to protect the antenna. However, since a reflection of the electromagnetic wave occurs on the wall surface of the radome, this influence must be suppressed. For this reason, JP09-046119A and JP10-200328A disclose radomes of the antenna device, in which a wall on the emitting side of the antenna is formed in a double-wall structure to cancel out the reflections between the wall surfaces, thereby improving the emission properties.
However, the structure of the radome disclosed in JP09-046119A and JP10-200328A cannot have a wide vertical angle range where the electromagnetic wave becomes a predetermined level or more (vertical radiation pattern). Therefore, if a ship where the antenna device is installed rocks by waves, it may not be able to transmit the electromagnetic wave of an effective level, stably in a target direction.